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Date:   Thu, 25 May 2023 13:46:40 +0000
To:     Alice Ryhl <aliceryhl@google.com>
From:   Benno Lossin <benno.lossin@proton.me>
Cc:     Miguel Ojeda <ojeda@kernel.org>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>,
        =?utf-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Will Deacon <will@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org,
        patches@lists.linux.dev, Andreas Hindborg <a.hindborg@samsung.com>
Subject: Re: [PATCH v2 4/4] rust: task: add `Send` marker to `Task`
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On 5/23/23 16:44, Alice Ryhl wrote:
> When a type also implements `Sync`, the meaning of `Send` is just "this
> type may be accessed mutably from threads other than the one it is
> created on". That's ok for this type.
>=20
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com>

Reviewed-by: Benno Lossin <benno.lossin@proton.me>

> ---
>   rust/kernel/task.rs | 10 ++++++++--
>   1 file changed, 8 insertions(+), 2 deletions(-)
>=20
> diff --git a/rust/kernel/task.rs b/rust/kernel/task.rs
> index 526d29a0ae27..7eda15e5f1b3 100644
> --- a/rust/kernel/task.rs
> +++ b/rust/kernel/task.rs
> @@ -64,8 +64,14 @@ macro_rules! current {
>   #[repr(transparent)]
>   pub struct Task(pub(crate) Opaque<bindings::task_struct>);
>=20
> -// SAFETY: It's OK to access `Task` through references from other thread=
s because we're either
> -// accessing properties that don't change (e.g., `pid`, `group_leader`) =
or that are properly
> +// SAFETY: By design, the only way to access a `Task` is via the `curren=
t` function or via an
> +// `ARef<Task>` obtained through the `AlwaysRefCounted` impl. This means=
 that the only situation in
> +// which a `Task` can be accessed mutably is when the refcount drops to =
zero and the destructor
> +// runs. It is safe for that to happen on any thread, so it is ok for th=
is type to be `Send`.
> +unsafe impl Send for Task {}
> +
> +// SAFETY: It's OK to access `Task` through shared references from other=
 threads because we're
> +// either accessing properties that don't change (e.g., `pid`, `group_le=
ader`) or that are properly
>   // synchronised by C code (e.g., `signal_pending`).
>   unsafe impl Sync for Task {}
>=20
> --
> 2.40.1.698.g37aff9b760-goog
>=20

--=20
Cheers,
Benno